The following invention relates to a driving network for a low-power TFEL panel using a symmetrical addressing technique.
Thin-film electroluminescent (TFEL) panels comprise a thin electroluminescent film sandwiched between orthogonally disposed row and column electrodes to form a matrix of pixels which may be selectively illuminated. Such panels are described generally in co-pending patent applications Serial Nos. 729,974 and 728,861 and assigned to the same assignee. An important consideration in the design of such panels is conservation of energy so that the panel may be made more compact with fewer and lighter components. Also, reduction of power consumption is a key factor in obtaining a better optical image, longer screen life, and portability.
One problem with conventional TFEL panels is their tendency to produce a latent image. This effect is due to the inability of the driving network to completely remove the luminescence of pixels illuminated in a previous frame of data. Usually a frame of data consists of the line-by-line illumination of pixels along a plurality of row electrodes which are sequentially charged beginning with the row at the top of the screen and proceeding to the bottom. As the pixels in one frame are illuminated, the luminescence of the pixels on the previous frame must be eliminated. This requires the use of what has been termed a "refresh" pulse which is a high-voltage pulse of polarity opposite that of the polarity of the illuminated pixels. The refresh pulse is thus used to "erase" an illuminated pixel which has been "written" with a high preconditioning row electrode voltage called a "write" voltage in combination with a column "modulation" voltage impressed upon those columns for which pixels in a particular row are to be illuminated. Contrast and latent image may be problems where the refresh pulse does not effectively eliminate the previous pixel charge. The resulting residual luminescence may create a latent image or a screen having lower contrast between lit and unlit pixels.
In addition, conventional driving architecture provides no means of recovering any of the energy from the panel and therefore there exist I.sup.2 R losses which are dissipated as heat. Heat, however, tends to decrease the display lifetime of the panel.
Several of the above-mentioned problems, in particular the problems with latent images, low contrast and lifetime of the panel have been partially rectified by the use of symmetric drive. Symmetric drive has been incorporated in a TFEL panel manufactured by Sharp Electronics, Model No. LJ512u05. The Sharp symmetric drive scheme utilizes pulses of alternating polarity on even and odd rows, respectively. For example, for a particular frame when even rows have a positive polarity, odd rows will have a negative polarity. This type of symmetric drive implementation leads, however, to a problem peculiar to TFEL panels. TFEL panels are, in part, piezoelectric. Shifting the polarity of adjacent pixels during the writing of a frame of data may at certain frequencies cause the panel itself to emit sound or "squeal." It is obviously undesirable for the panel to vibrate with an audible frequency.
A second problem with the aforementioned Sharp symmetric drive technique is the need for complimentary pairs of integrated circuit chips for the row drivers. Since the row drivers alternate between negative and positive polarity, both N-type and P-type FET's must be used. This doubles the number of components needed to drive the rows over conventional non-symmetric addressing techniques.
Previous drive schemes also provided no means for recovering power from the TFEL panel. As mentioned above, all power heretofore has been dissipated in the form of heat and/or light. A desirable feature in such panels would be the conversion of what are normally I.sup.2 R losses into useful power that could be recovered from the panel. It is known that there exist power recovery techniques for use with AC TFEL displays. For example, in "A Low Power Drive Scheme for AC TFEL Displays," Society For Information Display Digest, 1985, by Marvin L. Higgins, a series resonant circuit for driving the column electrodes of a TFEL display is described. Since the display panel is largely capacitive, the column drive network provides an inductive reactance such that the circuit formed by the column driver and the TFEL panel becomes a series-connected RLC circuit. Drive pulses are provided at the resonant frequency of the circuit but are clamped at predetermined voltage levels to prevent the circuit from oscillating. In this way power provided to the largely capacitive panel may be recovered and stored as energy in the primarily inductive series resonant circuit. This approach, however, has not been used on symmetric drive TFEL panels.
A further consideration is the efficiency of the circuit components needed to drive such panels. A high-voltage switching power supply is usually needed to provide the column and row drivers with high-voltage pulses, and typically such pulses may be on the order of +210 to -160 volts for the row drivers and +50 volts for the column drivers. These switching power supplies, however, are only approximately 75% efficient and require components that increase the physical size of the circuit boards necessary to drive the panel.
What is needed, therefore, is a low-power TFEL display panel using symmetric drive addressing while taking advantage of series resonant power recovery techniques and at the same time reducing circuit board size and complexity.